Considering the necessity for standardization of toxicity tests and the need for simple and reliable routine tests, the Artemia Reference Center at the State University of Ghent in Belgium has developed a short term standard toxicity test with Artemia nauplii. This simple and inexpensive bioassay which can be used as a screening test to determine the toxicity of chemicals to aquatic organisms, has been discussed in detail during a special workshop devoted to toxicity tests with brine shrimp, at the occasion of the "International Symposium on the brine shrimp Artemia salina"(Corpus Christi, Texas, USA; August 1979). Besides a few minor modifications, the test methodology was acceptable to the specialists present, who recommended that an intercalibration exercise be organized at the international level, in order to determine the reliability and the reproducibility of the test procedure. Such an exercise has recently been organized in North America under the joint supervision of the Artemia Reference center in Ghent, Belgium and the Toxicology Section of the Freshwater Institute in Winnipeg, Canada. An analogous exercise was conducted in Europe by the Artemia Reference Center under the sponsorship of the Commission of the European Economic Communities. The result of the latter ring test in which 59 laboratories from 11 countries participated are presented and commented upon. The revised version of the standardized ARC-test, resulting from the intercalibration exercise, is included.